This invention generally relates to ultrasound imaging for the purpose of medical diagnosis. In particular, the invention relates to methods for imaging tissue and blood flow by detecting ultrasonic echoes reflected from a scanned region of interest in a human body.
A conventional ultrasound image is composed of multiple image scan lines. A single scan line (or a small, localized group of scan lines) is acquired by transmitting focused ultrasound energy at a point in the region of interest and then receiving the reflected energy over time. The focused transmit energy is referred to as a transmit beam. During the time after transmit, one or more receive beamformers coherently sum the energy received by each channel, using dynamically changing phase rotation or time delays to produce peak sensitivity along the desired scan lines at ranges proportional to the elapsed time. The resulting focused sensitivity pattern is referred to as a receive beam. A scan line""s resolution results from the directivity of the associated transmit and receive beam pair.
In a typical ultrasound imaging system, the outputs of the beamformer channels are coherently summed and envelope-detected to form a respective intensity value for each sample volume in the object region or volume of interest. These intensity values are log-compressed, scan-converted and then displayed as pixels in an image frame of the anatomy being scanned. Successive image frames are typically averaged prior to scan conversion with the goal of reducing electronic and speckle noise.
The design of medical ultrasonic imaging equipment requires a careful compromise between image quality and frame rate. Some techniques are available which can improve frame rate without significantly degrading image quality. For example, state-of-the-art ultrasound imagers allow for the simultaneous acquisition of multiple receive scan lines for a single transmit firing or apply advanced interpolation schemes which reduce the required number of receive scan lines. However, even after exploiting these methods there is still a need to improve frame rate without significantly degrading image quality.
To achieve the desired image quality, the ultrasound imaging system compromises the frame rate in two ways. First, a large number of transmit focal points are used to obtain good spatial resolution. The best resolution can only be achieved when the transmit beam is tightly focused, that is, when the transmit aperture operates at a low f-number. However, a tight transmit focus results in a short depth of field and thus requires a large number of focal points to maintain the transmit focus over the region of interest.
The second frame rate compromise (at least to the perceived frame rate) occurs in the frame-averaging or image persistence processing. This processing applies a FIR or IIR filter to successive acoustic frames at each image pixel to reduce electronic and speckle noise. If the image is exactly stationary, then such processing reduces the electronic noise while preserving the desired acoustic image. Because electronic noise is reduced, the depth to which anatomy can be perceived is increased. If the target is moving slightly with respect to the transducer, then a beneficial speckle averaging also occurs. The speckle structure is typically on a finer scale than the anatomical features. Thus, with small target motions, frame-averaging reduces the speckle variation while retaining the anatomical structures. Larger target motions, however, blur the anatomical structures and reduce the diagnostic usefulness of the frame-averaged image.
Conventional ultrasound imaging systems provide a selection of xe2x80x9cpresetsxe2x80x9d which control the image quality to frame rate compromise on an application-specific basis. Additionally, the user has the option of modifying these presets by changing imaging parameters such as number of transmit focal points, displayed image size, scan line density, degree of frame-averaging and so on. However, once the system is configured, it remains in this operating condition until the user makes another parameter adjustment.
A low imaging frame rate is objectionable when the displayed structures move rapidly within the image. However, the importance of a high frame rate is reduced when the target is stationary. Typically, the sonographer moves the transducer in a search phase until the best view of the region of interest is found. This region is then examined in an evaluation phase for some type of lesion or other abnormality. During the search phase, high frame rate is often more important than image quality, while image quality is more relevant than frame rate in the evaluation phase. There is a need for a method of adaptively adjusting the frame rate in accordance with this understanding.
The invention disclosed herein approaches the image quality and frame rate compromise in a different way. It is based on the recognition that while image quality and frame rate are both important, they are often not required simultaneously. During the search phase, when the transducer is moving and large target motions result, high frame rate is more important and image quality can be somewhat reduced. In the search mode the user cannot take advantage of an optimal image quality because the target moves too rapidly. During the evaluation phase, when the target is stationary or moving slowly, the user concentrates on the image details and requires optimal image quality. High frame rate is less important since no motion artifacts are introduced. The invention disclosed herein dynamically optimizes the frame rate using an estimate of the target motion.
The frame rate depends upon the number of transmit firings in an image frame, i.e., the number of transmit focal zones in each scan line and the number of scan lines in the image frame. By reducing the number of firings, the frame rate is increased. Additionally, the frame rate perceived by the operator depends on the degree of frame-averaging, or image persistence. The dynamic frame rate adjustment in accordance with the preferred embodiment comprises two steps. First, the target motion is estimated, and then this estimate is used to control the number of transmit firings per frame and/or the degree of frame-averaging.
The motion estimation can be as complex as a two-dimensional cross-correlation between frames. However, the method presented here requires only a low accuracy in the estimate of the magnitude of the motion and does not require any information about the direction of the motion. Therefore in accordance with the most preferred embodiments of the invention, the motion is estimated by measuring pixel brightness variations. This estimate can be calculated using all the pixels in the image frame (frame-to-frame estimation), using pixels within a selected region with the image frame (region-to-region estimation), or between individual scan lines within the image frame (line-to-line estimation). Alternatively, motion can be detected by measuring a Doppler signal.
In accordance with one preferred embodiment of the invention, the degree of frame-averaging is adjusted as a function of the estimated target motion during operation of the imaging system. In accordance with another preferred embodiment, the number of transmit firings per frame is adjusted as a function of estimated target motion. Other imaging parameters, such as the size of the transmit aperture and the transmit excitation frequency can also be adjusted as a function of estimated target motion.
The user is presented with an image that smoothly tracks the target motion yet provides optimal image quality when the target is stationary or nearly stationary.